1. Field of the Invention
The present invention relates to a current source type current output circuit and, more particularly, to a circuit for applying to a load a constant output current which is proportional to an input voltage or an input current, with no regard to the character of the load.
2. Description of the Prior Art
A current source type current output circuit is disclosed in, for example, Michio Okamura "DESIGN OF OP AMP CIRCUIT (REVISED)," published Aug. 10, 1973 by CQ Publishing Co., Ltd., Tokyo, Japan, pp. 87 and 88. The principle of such a prior art current source type current output circuit is shown in FIG. 2. The circuit of FIG. 2 includes a voltage-input voltage-output type operational amplifier (op amp) A1 which is adapted to produce an output voltage by multiplying an input voltage by a predetermined gain A1. The output B of the op amp A1 is connected to an output terminal C of the circuit via a resistor R11. Connected to the output terminal C is a circuit L1 which constitutes a load of the circuit. The inverting input D of the op amp A1 is connected on one hand to the output terminal B of the op amp A1 via a resistor R13 and on the other hand to an input signal voltage source V12 via a resistor R14. Likewise, the non-inverting input E of the op amp A1 is connected on one hand to the output terminal C of the current output circuit via a resistor R12 and on the other hand to ground via a resistor R15.
In the prior art current output circuit shown and described, assuming that the absolute value of the gain A1 of the op amp A1 is sufficiently large, and that the resistors R11, R12, R13, R14 and R15 are related to each other as represented by: EQU R14.multidot.(R11+R12)=R13.multidot.R15 Eq.(1)
then, the output current I13 of the circuit is produced by: ##EQU1## Hence, an output current I13 which is proportional to an input signal voltage V12 is obtained. Although the characteristic of the load L1 of the current output circuit may be changed to in turn change the potential V11 which appears on the output terminal C, the Eq. (2) is maintained so long as the relation as represented by the Eq. (1) is satisfied. In this condition, the current output circuit succeeds in feeding a constant current to the load L.
However, a drawback with the prior art current source type current output circuit discussed above is that as many as five resistors having accuracy which satisfies the previously stated Eq. (1) are installed. As regards an integrated circuit, since such resistors with substantial accuracy occupy a substantial volume, it is impractical to configure the prior art current output circuit in a miniature and dense arrangement.